This invention relates to a thermosetting resin composition and its uses, and particularly to a thermosetting resin composition for a printed circuit board having a good flame retardness and a printed circuit board using the resin composition.
Heretofore, phenol resin, epoxy resin, polyimide resin, etc. have been used as resin materials for multi-layered printed circuit boards to be used in electronic computers, etc. Recent large-scale electronic computers require a high speed computation and thus strict characteristics are required for the multi-layered printed circuit boards for this purpose and development of new materials for the multi-layered printed circuit boards is now in progress.
Generally, the computing speed of an electronic computer largely depends on the signal transmission speed of the circuit on a multi-layered printed circuit board used in the electronic computer. The signal transmission speed of the circuit also depends on the dielectric constant of the insulating layer. The lower the dielectric constant, the higher the signal transmission speed. Thus, the computing speed of an electronic computer can be improved by making a printed circuit board from materials of low dielectric constant.
Polytetrafluoroethylene (PTFE), polybutadiene, etc. are known as materials of low dielectric constant. Furthermore, cyanate compounds (U.S. Pat. No. 4,559,3999) or isocyanate compounds (U.S. Pat. No. 4,353,7691) having a structure of low polarization based on a aromatic group or alicylic group or a combination thereof on the main chain are also known as materials of low dielectric constant.
PTFE belongs to a thermoplastic resin and has problems such as poor dimensional stability and throughhole reliability, when used in multi-layered printed circuit boards. Still furthermore, no appropriate solvent is available for PTFE and thus the lamination bonding has been relied only on a heat fusion pressing.
Owing to a high melting temperature such as 250.degree. C. to 350.degree. C., PTFE has a poor workability and problems in the handling, as compared with the conventional epoxy resin, etc.
As to the polybutadiene, on the other hand, resin materials based on a combination of 1,2-polybutadiene having double bonds on the side chains with a cross-linking type flame retardant based on bifunctional monomers have been developed [Japanese Patent Application Kokai (Laid-open) No. 55-126451]. When a lower molecular weight polymer is used in view of the impregnation property toward fibrous substrate materials, the tackiness of the resulting prepreg is so high that it is difficult to cut or preserve the prepreg and also an adverse effect is given on the lamination-bonding workability. When a higher molecular weight polymer is used in view of a low tackiness or tackfreeness of a prepreg, the impregnation property toward fibrous substrate materials is deteriorated due to the higher varnish viscosity and it is not easy to prepare a prepreg. That is, the resulting printed circuit boards have no satisfactory qualities. Furthermore, the curing reaction is a cross-linking reaction based on radical polymerization and thus the reaction rate is so high that its control is not easy. Still furthermore, polybutadiene is liable to form cracks, when molded, because of a high curing shrinkage and has low mechanical strength and heat resistance and a low adhesion to copper foils that form circuits.
The cyanate compounds or isocyanate compounds are trimerized in the presence of a catalyst to form cured products of high cross-linking density. The cured products have a low dielectric constant and good dimensional stability and heat resistance, but has no satisfactory flame retardness as an important characteristic of printed circuit boards.
In order to improve the foregoing disadvantages, Japanese Patent Application Kokai (Laid-open) No. 61-243844 (=U.S. patent application Ser. No. 86/854507; EP Patent Application No. 86105550.7) proposes a thermosetting resin composition comprising a prepolymer of poly(p-hydroxystyrene) derivative and 1,2-polybutadiene or its derivative, a laminated board using the resin composition, and a process for producing the laminated board, and Japanese Patent Application Kokai (Laid-open) No. 62-192406 (=U.S. patent application Ser. No. 87/15325; EP Patent application No. 87102069.9) proposes a flame-retarding resin composition comprising a prepolymer of poly(p-hydroxystyrene) derivative as an essential component, and a prepreg and a laminated board using the resin composition, but the laminated boards made from these resin compositions still have problems in the moisture absorption, chronological change and mechanical characteristics at an elevated temperature.